Transient documents and photochromic paper for transient documents have already been reported in U.S. patent application Ser. No. 10/835,518 and U.S. patent application Ser. No. 10/834,722, where images printed on photochromic paper can be read for a few hours under room light conditions, and the transient documents self-erase with no effort from the user and are ready to be printed again with new images the next day. The resulting blank sheet of paper is ready to be printed again with new information, and the paper may be reusable many times. Although there are other available technologies such as liquid crystals, electrophoretics, or gyricon for providing transient documents, none of these technologies can provide a document that really feels like paper.
A different application for photochromic based media is for reimageable documents such as, for example, electronic paper documents. Reimageable documents require information to be kept for as long as the user wants, then the information can be erased or the reimageable document can be re-imaged using an imaging system with different information. Written images on electronic paper may be readable for longer period of time such as, for example, many days or weeks.
Buncel et al. (J. T. C. Wojtyk, P. M. Kazmaier, E. Buncel. J. Chem. Soc. Chem. Comm. 1703, (1998)) reported life-times of at least several days for solutions in acetone of spiropyrans modified with chelating groups in the presence of metallic cations. As shown in Equation 1 below, the metal cation Mn+ can stabilize the open merocyanine form through chelation. For comparison, a classical spiropyran molecule, containing no chelating groups, is stable in its colored form for only a few minutes.